In the fabrication of some semiconductor wafers, thin films of tungsten are deposited to provide gate interconnections, contacts, vias, and contact barrier metals. Tungsten provides material for interconnect applications as a result of its low resistance, low stress, excellent conformal step coverage and because its thermal expansion coefficient closely matches that of silicon. Typically, the tungsten films are formed both in selective and blanket deposition modes.
Tungsten thin films are usually deposited using a chemical vapor deposition (CVD) process, wherein solid films are formed on an integrated circuit wafer by the chemical reaction of vapor phase chemicals (reactants) that contain the required constituent gases. Three major CVD processes exist: atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), and plasma-enhanced CVD (PECVD). APCVD and LPCVD systems are characterized by the requisite pressure for the deposition. Typically, these systems use thermal energy to promote chemical reactions responsible for the film deposition. PECVD systems, however, are characterized by pressure and by its method of energy input. PECVD systems do not rely solely on thermal energy, but instead use a radio-frequency (RF) induced glow discharge plasma to transfer energy into the reactant gases, allowing the integrated circuit wafer to remain at a lower temperature than in APCVD or LPCVD processes.
Generally, in CVD systems, the films are deposited in batch reactors which accommodate a large number of wafers at the same time. Single wafer systems are also available which accommodate one wafer at a time.
In the course of many semiconductor device processing steps, unwanted deposits and contaminants are formed in the chamber of the processing equipment. As these deposits and contaminants accumulate in successive runs, they can interfere with the deposition process, causing particle and chemical contamination of the wafers, thereby resulting in a low die yield. Thus, frequent cleaning of the chamber wall is required. Specifically with regard to thin tungsten films, their deposition by the reduction of tungsten hexafloride (WF.sub.6) vapor can be adversely affected by the accumulation of reaction by-products, such as hydrogen, fluorine, and fluorine-containing compounds, in the reaction chamber. These by-products are generated during the deposition reaction as well as subsequent chamber cleaning processes, and can cause excessive non-uniformity and variability of critical film properties, such as thickness, resistivity, and reflectivity.
In the prior art, chamber cleaning usually encompasses the partial disassembly of the process chamber and cleaning the chamber with corrosive and toxic chemicals. In certain tungsten deposition processes, film properties (specifically within wafer thickness non-uniformity) were found to reach unacceptable levels after as few as fifty wafers. When this occurs, the chamber must then be cooled, vented, opened, manually cleaned with wet chemicals, realigned, sealed, pumped, leak-checked, heated and re-qualified so that the tungsten deposition process could be subsequently performed. This cleaning procedure normally requires approximately eight hours of equipment time. This cleaning procedure causes long equipment downtimes, resulting in as much as a sixty percent downtime for the chamber cleaning alone. High labor costs and loss of process repeatability due to chamber disassembly and reassembly, the breakage and/or degregation of the chamber and other equipment parts due to the disassembly and reassembly and handling of the chamber and other process hardware during the cleaning process are also drawbacks to this cleaning procedure. Furthermore, the chemical contamination of the chamber by liquids used in cleaning and handling, along with the safety hazards associated with the use of corrosive and toxic chemicals, are also drawbacks.
As will be shown, the present invention provides a method of cleaning the chamber that is conducted in the same vacuum environment in which the film is deposited (in-situ). The present invention comprises a technique for scavenging or removing the by-products from the reaction chamber without the need for an extensive and time-consuming manual cleaning procedures. The present invention also reduces the downtime resulting from chamber cleaning to approximately ten percent.